This invention relates generally to systems for monitoring ion concentrations, and more particularly, to a system for monitoring sodium ion concentrations in fluids, including fluids contained within human beings.
Throughout a variety of industries, and particularly in various medical fields, there is a need for simple and economical systems which can monitor accurately the concentration of specific ions in fluids. The need for an economical and accurate system which can monitor the concentration of sodium ions is particularly acute in certain medical procedures, such as during transurethral resection of the prostate (TURP).
It is very common in middle aged and elderly men that the prostate gland enlarges and compresses the urethra so as to impede the passage of urine. It has been suggested that approximately 40% of men over 65 years of age have some difficulty in passing urine. The condition is progressive whereby the continued enlargement of the prostate gland occasionally blocks entirely the urethral passageway. This emergency condition, where the man cannot urinate at all, usually requires resection of some or all of the prostate gland to remove the obstruction. Resection can be achieved either by surgical incision or by the passage of an optical cutting instrument through the urethra to resect the gland from the inside.
TURP has many advantages over transdermal surgery. These include, for example, lower expense and reduced hospital stay, patient acceptability, as well as reduction in mortality associated with the ease of sputum clearance from the chest when coughing is not restricted by a painful incision. In addition, early ambulation is believed to contribute to relatively infrequent venous thrombosis and pulmonary embolism.
TURP is usually performed with a fluid-irrigated endoscope. The requirements of optical clarity and non-conduction of the resecting diathermy current by anything other than the tissue to be cut, demand that a non-conducting, and hence sodium-free irrigating fluid be used. The irrigating fluid, however, may have a hydrostatic pressure sufficient to force it into prostatic venous sinuses which are opened during resection of the gland. On the average, in an uncomplicated transurethral resection, the amount of irrigating fluid absorbed is probably between 100 and 1000 ml. However, in certain situations, patients may absorb more than 10 liters of fluid during TURP, and such patients therefore develop the complete TURP syndrome. Although most of the fluid is absorbed during the TURP procedure itself, an additional amount of the irrigating fluid is absorbed after the procedure during the continuous irrigation of the bladder generally conducted after surgery. The resulting increase in fluid load, hyponatremia and hemolysis, which depends upon the nature of the fluid, produce the TURP syndrome. The clinical features of fluid overload and hyponatremia are seen after the circulation has sustained the overload, and are therefore almost useless for warning of impending catastrophe.
Although there are present in the patient during anesthesia certain symptoms and signs associated with the syndrome, this syndrome stands alone as being so commonly encountered with so little to warn of its danger. The symptoms are difficult to interpret since they are to a large extent dependent upon the anesthesia which is used during TURP. For example, patients who are under the influence of spinal anesthesia will show symptoms of confusion, hypertension, nausea, restlessness, and increased central venous pressure. However, in an unconscious patient, the nausea, disorientation, and restlessness are not seen. In fact, some of the other signs of the syndrome are delayed by assisted ventilation. Thus, increasing systolic and diastolic pressures, and increased central venous pressure, which is usually associated with abdominal distention and tightness, are the only signs apparent in a generally anesthetized patient. These observations of signs of acute circulatory overload, as well as the associated electrolyte disturbance, are insufficient warning systems for the care of a patient who may well be old and frail. Other alternatives which have been proposed, such as the monitoring of changes in transthoracic impedance have been too cumbersome, unreliable, or expensive for routine use.
One known system for detecting hemodilution depends upon the taking of serial samples for estimation of some marker of dilution, such as sodium concentration, hematocrit, or osmolarity, will require very many samples at short intervals if prompt detection of early changes is to be possible. It is clear that there is a need for a continuous method of analysis which allows continuous monitoring of hemodilution. In this regard, the plasma sodium concentration is an excellent marker of dilution because it is a familiar index of hydration to physicians, and also, since its ions are electrically conductive, it cannot be present in any irrigating fluid.
The foregoing is merely illustrative of a specific need in the present state of the art. There is additionally a need to monitor, in medical and other fields, a variety of other ion concentrations, such as K.sup.+, H.sup.+, and Ca.sup.+. The need to monitor such concentrations accurately is present in the manufacture of food and drugs, the brewing industry, the production of fertilizers, the treatment of sewage, and other technical fields.
It is, therefore, an object of this invention to provide a simple and economical system for measuring ionic concentrations.
It is another object of this invention to provide an ion concentration monitoring arrangement which uses membranes rather than field effect transistors.
It is also an object of this invention to provide an arrangement for monitoring the concentration of ions and which can be miniaturized sufficiently for insertion into a living being.
It is additionally an object of this invention to provide a system which can easily be adapted to monitor various types of ionic concentrations.
It is a further object of this invention to provide an ionic concentration monitoring system which can be adapted to be selective and therefore particularly responsive to a particular ion.
It is still another object of this invention to provide an ion concentration monitoring system which does not employ hazardous electrical potentials.
It is a yet further object of this invention to provide an ion sensor which is easy to fabricate.
It is also a further object of this invention to provide a system which reacts quickly to changes in ionic concentration.
It is yet another object of this invention to provide an ionic concentration monitoring system which is electrically stable and has low drift.